v0.3.0

Sell as many days as a crawler wants in one payment. ?days=N quotes N terms; the days a proof buys are read off the value it authorizes (a whole multiple of the day price, up to maxDays, default 30); the pass expires that many days out. Receipt carries days/minutes; the 402 pass object carries days, total, maxDays, buyDays; onSale gets days/totalCents.